I was just curious what the purpose of relays serve for offroad lights
Thanks for any and all info

 

So the way i understand it, for just a switch, the load of the power going to the lights goes through the switch and if it were to heat up and catch on fire, it would be in your face. With a relay, it takes the load off of the switch and through the relay, which is usually built to handle more power and won't catch fire (but if it did it would be in the engine away from you).

 

He is correct. The relay keeps the power load off the switch, and they are designed to handle the load.

 

Ok that makes plenty if sense...the same then should really be done for anything wired up to a switch?

 

Well it depends on how much power is going through it, but probably. In-line fuses are also a good safety precaution.

 

Yep, and in addition to what these guys said, the power follows a more direct 'path' to whatever you're running. Aside from melting switches, one of the gotchas with low-voltage DC is that the further you run wire, the larger it needs to be to keep the voltage up.

The switch just toggles whether the relay is on or off, which doesn't draw much juice. For a long run of battery -> switch -> relay you can get away with using cheap, thin gauge wire. Then, for the battery -> relay -> light you can use the appropriate gauge, say 10 AWG for some, and get plenty of power to your light safely.

Compare this to the factory headlight setup, where power goes battery -> switch -> lights. The wire is way too thin for how far it runs, and as a result the lights don't get the voltage they need (or are able to draw the current they need) to burn as brightly as they should. If you simply replaced all of the wire with some heavier copper they'd be a lot brighter... though your switch might still melt lol

 

As a thumb rule, I'll wire anything that draws more than 500mA (one-half Ampere) through a relay.

This allows me to:
- Have a broader selection of switches available to use (since switches are rated by current capacity - "ampacity")
- Use less heavy-gage wire.
- Usually, I don't have to run a high-current lead through a bulkhead (which I don't like to do)
- Run a more direct high-current lead to the device (which reduces conductor loss)

And probably a couple of other things.

As far as fusing - yes, you still want to install a fuse. The fuse should go in between the power source and the relay, it should be the first device that the power encounters that is not a wire.

 

There are various instances when, even if the current draw doesn't demand a relay, it may be beneficial to use one.

Most aftermarket automotive relays are rated at 30 or 40 amps, which would be 360 or 480 watts.

Most aftermarket switches are rated to carry at least 10 amps, with many rated at 20.

As a general rule, I'll use a relay on anything that uses more than 10 amps of power. I have several LED lights that only pull a few watts, or less than 2 amps, and I'll run those without relays all day long. There's just no need to get that involved with all of the wiring when your amperage load is so low.

 

How much power does it take to trip the relay? I'm under the impression it's litterally milliamps rather than amps?

You would use a 1 amp fuse on your switch right?

 

Relay windings are usually 80-100 ohms and draw around 150 milliamps. The coil has an inductive voltage spike when switched off which pits the switch contacts and can rarely cause RFI issues with other electronics. Suppressed relays will have a diode or resistor in parallel with the winding and surpress the spike. If you have a unsupressed relay, you can put a 1/4 watt 500 ohm resistor in parallel with the winding or put a diode across with correct polarity.

1 amp fuse would work, a half amp fuse would be safer.